A head-up display (hereinafter referred to as a “HUD”) for a vehicle displays driving support information as a virtual image in front of a windshield as viewed from the driver. The driving support information includes, for example, a speed indication, navigation information and so on. The driver can visually recognize a scene in front of the vehicle and the driving support information in a superimposed state. This allows the driver to shorten the time for moving his/her line of vision or the time for focal adjustment, in the driving of the vehicle. Accordingly, the HUD is capable of lessening fatigue of the driver and improving safety.
To further shorten the time for the focal adjustment in driving, a virtual image display distance needs to be brought closer to a focal distance of the driver's eyes. The virtual image display distance is a distance from the driver to the virtual image. The focal distance of the driver's eyes is a distance from the driver to a position the driver is watching. However, the position the driver is watching changes according to traveling speed and so on. Thus, there has been proposed a technology that changes the virtual image display distance according to the vehicle's traveling speed and so on (see Patent Reference 1, for example).
In the technology disclosed in the Patent Reference 1, a scan means for scanning a light beam from a light source, a screen on which the light beam scanned by the scan means forms an image and a projection means for projecting the image formed on the screen are provided, and the virtual image display distance is changed by moving the screen.